Index: include/llvm/Transforms/Utils/SimplifyLibCalls.h
===================================================================
--- include/llvm/Transforms/Utils/SimplifyLibCalls.h
+++ include/llvm/Transforms/Utils/SimplifyLibCalls.h
@@ -160,6 +160,7 @@
   Value *optimizeStrRChr(CallInst *CI, IRBuilder<> &B);
   Value *optimizeStrCmp(CallInst *CI, IRBuilder<> &B);
   Value *optimizeStrNCmp(CallInst *CI, IRBuilder<> &B);
+  Value *optimizeStrNDup(CallInst *CI, IRBuilder<> &B);
   Value *optimizeStrCpy(CallInst *CI, IRBuilder<> &B);
   Value *optimizeStpCpy(CallInst *CI, IRBuilder<> &B);
   Value *optimizeStrNCpy(CallInst *CI, IRBuilder<> &B);
Index: lib/Transforms/InstCombine/InstCombineCalls.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -4223,7 +4223,8 @@
     annotateAnyAllocSite(Call, &TLI);
 
   if (isAllocLikeFn(&Call, &TLI))
-    return visitAllocSite(Call);
+    if (Instruction *I = visitAllocSite(Call))
+      return I;
 
   bool Changed = false;
 
Index: lib/Transforms/Utils/BuildLibCalls.cpp
===================================================================
--- lib/Transforms/Utils/BuildLibCalls.cpp
+++ lib/Transforms/Utils/BuildLibCalls.cpp
@@ -842,6 +842,13 @@
                      B.getInt8PtrTy(), castToCStr(Ptr, B), B, TLI);
 }
 
+Value *llvm::emitStrDup(Value *Ptr, IRBuilder<> &B,
+                        const TargetLibraryInfo *TLI) {
+  LLVMContext &Context = B.GetInsertBlock()->getContext();
+  return emitLibCall(LibFunc_strdup, B.getInt8PtrTy(), B.getInt8PtrTy(),
+                     castToCStr(Ptr, B), B, TLI);
+}
+
 Value *llvm::emitStrChr(Value *Ptr, char C, IRBuilder<> &B,
                         const TargetLibraryInfo *TLI) {
   Type *I8Ptr = B.getInt8PtrTy();
Index: lib/Transforms/Utils/SimplifyLibCalls.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -531,6 +531,21 @@
   return nullptr;
 }
 
+Value *LibCallSimplifier::optimizeStrNDup(CallInst *CI, IRBuilder<> &B) {
+  Value *Src = CI->getArgOperand(0);
+  ConstantInt *Size = dyn_cast<ConstantInt>(CI->getArgOperand(1));
+  uint64_t SrcLen = GetStringLength(Src);
+  if (SrcLen && Size) {
+    annotateDereferenceableBytes(CI, 0, SrcLen);
+    if (!Size->isNullValue()) {
+      if (SrcLen <= Size->getZExtValue() + 1)
+        return emitStrDup(Src, B, TLI);
+    }
+  }
+
+  return nullptr;
+}
+
 Value *LibCallSimplifier::optimizeStrCpy(CallInst *CI, IRBuilder<> &B) {
   Value *Dst = CI->getArgOperand(0), *Src = CI->getArgOperand(1);
   if (Dst == Src) // strcpy(x,x)  -> x
@@ -2716,6 +2731,8 @@
       return optimizeStrLen(CI, Builder);
     case LibFunc_strpbrk:
       return optimizeStrPBrk(CI, Builder);
+    case LibFunc_strndup:
+      return optimizeStrNDup(CI, Builder);
     case LibFunc_strtol:
     case LibFunc_strtod:
     case LibFunc_strtof:
Index: test/Transforms/InstCombine/strndup.ll
===================================================================
--- test/Transforms/InstCombine/strndup.ll
+++ test/Transforms/InstCombine/strndup.ll
@@ -0,0 +1,67 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -instcombine -S | FileCheck %s
+
+@hello = constant [6 x i8] c"hello\00"
+@null = constant [1 x i8] zeroinitializer
+
+declare i8* @strndup(i8*, i32)
+
+define i8* @test1() {
+; CHECK-LABEL: @test1(
+; CHECK-NEXT:    [[RET:%.*]] = call i8* @strndup(i8* dereferenceable(1) getelementptr inbounds ([1 x i8], [1 x i8]* @null, i64 0, i64 0), i32 0)
+; CHECK-NEXT:    ret i8* [[RET]]
+;
+  %src = getelementptr [1 x i8], [1 x i8]* @null, i32 0, i32 0
+  %ret = call i8* @strndup(i8* %src, i32 0)
+  ret i8* %ret
+}
+
+define i8* @test2() {
+; CHECK-LABEL: @test2(
+; CHECK-NEXT:    [[RET:%.*]] = call i8* @strndup(i8* dereferenceable(6) getelementptr inbounds ([6 x i8], [6 x i8]* @hello, i64 0, i64 0), i32 4)
+; CHECK-NEXT:    ret i8* [[RET]]
+;
+  %src = getelementptr [6 x i8], [6 x i8]* @hello, i32 0, i32 0
+  %ret = call i8* @strndup(i8* %src, i32 4)
+  ret i8* %ret
+}
+
+define i8* @test3() {
+; CHECK-LABEL: @test3(
+; CHECK-NEXT:    [[STRDUP:%.*]] = call dereferenceable_or_null(6) i8* @strdup(i8* getelementptr inbounds ([6 x i8], [6 x i8]* @hello, i64 0, i64 0))
+; CHECK-NEXT:    ret i8* [[STRDUP]]
+;
+  %src = getelementptr [6 x i8], [6 x i8]* @hello, i32 0, i32 0
+  %ret = call i8* @strndup(i8* %src, i32 5)
+  ret i8* %ret
+}
+
+define i8* @test4() {
+; CHECK-LABEL: @test4(
+; CHECK-NEXT:    [[STRDUP:%.*]] = call dereferenceable_or_null(6) i8* @strdup(i8* getelementptr inbounds ([6 x i8], [6 x i8]* @hello, i64 0, i64 0))
+; CHECK-NEXT:    ret i8* [[STRDUP]]
+;
+  %src = getelementptr [6 x i8], [6 x i8]* @hello, i32 0, i32 0
+  %ret = call i8* @strndup(i8* %src, i32 6)
+  ret i8* %ret
+}
+
+define i8* @test5() {
+; CHECK-LABEL: @test5(
+; CHECK-NEXT:    [[STRDUP:%.*]] = call dereferenceable_or_null(6) i8* @strdup(i8* getelementptr inbounds ([6 x i8], [6 x i8]* @hello, i64 0, i64 0))
+; CHECK-NEXT:    ret i8* [[STRDUP]]
+;
+  %src = getelementptr [6 x i8], [6 x i8]* @hello, i32 0, i32 0
+  %ret = call i8* @strndup(i8* %src, i32 7)
+  ret i8* %ret
+}
+
+define i8* @test6(i32 %n) {
+; CHECK-LABEL: @test6(
+; CHECK-NEXT:    [[RET:%.*]] = call i8* @strndup(i8* getelementptr inbounds ([6 x i8], [6 x i8]* @hello, i64 0, i64 0), i32 [[N:%.*]])
+; CHECK-NEXT:    ret i8* [[RET]]
+;
+  %src = getelementptr [6 x i8], [6 x i8]* @hello, i32 0, i32 0
+  %ret = call i8* @strndup(i8* %src, i32 %n)
+  ret i8* %ret
+}